The present invention relates generally to methods and apparatus for removing entrained gas bubbles from a liquid and; more particularly to methods and apparatus for dissolving and separating entrained gas bubbles in a moving stream of a liquid by exposing the liquid to an ultrasonic field; and most particularly to a method for increasing the effectiveness of ultrasonic debubbling apparatus.
There are a variety of emulsions, suspensions, pastes, and high viscosity liquids used in the manufacture of or which become part of a variety of products in the chemical, pharmaceutical, food product, and photographic industries. These emulsions, suspensions, pastes, and high viscosity liquids often contain entrained air or gases present in the form of small bubbles. Often this air or gas, particularly in the case of entrained bubbles, is detrimental to the final product being produced. For example, in the case of photographic emulsions containing bubbles, the quality of the films or photographic papers produced is greatly impaired, giving rise to coated defects making the photographic materials unusable.
It is known to remove gas bubbles from solutions, emulsions, and other liquid compositions by exposing them to an imposed ultrasonic energy field. In such a field, large entrained gas bubbles are caused to coalesce and rise into a gas trap. Small bubbles may be collapsed and the gas driven into solution, depending upon the size of the bubble and the degree of gas saturation of the liquid composition. Apparatus for debubbling generally includes a metal vessel or tube containing a metal horn extending through an end wall of the vessel; one or more crystal transducers resonantly responsive to an imposed ultrasonic RF signal and bonded and/or bolted to the external end of the horn; and an RF signal generator of the proper frequency. Typically, debubbling apparatus in the art of preparing photographic emulsions are operated at ultrasonic frequencies between 25 kHz and 40 kHz.
An apparatus which is typically used in the photographic industry for de-bubbling photographic emulsions is an end cap round ultrasonic bubble eliminator, typically referred to as an ECR. The ECR includes a transducer horn assembly (hereinafter referred to as a xe2x80x9cTHAxe2x80x9d) which is an electromechanical device that converts electrical vibration to mechanical vibration. One particular ECR, with its component THA, is taught in U.S. Pat. No. 5,373,212 to Beau, hereby incorporated herein by reference. In the operation of an ECR, an alternating voltage is applied to a ceramic disc of the THA, which, as a result, generates mechanical vibration. This mechanical vibration assists in the debubbling of the photographic emulsions flowing through the ECR. Beau teaches a debubbling device wherein an ultrasonic power supply regulates power output to a predetermined constant level. The output of the generator is automatically adjusted to maintain a nominal power level, for example, 40 watts, in the face of changing load conditions that would otherwise cause the power to change in the absence of this feature. This is referred to in the prior art as a xe2x80x9cconstant powerxe2x80x9d setting of the generator.
U.S. Pat. No. 5,853,456 to Bryan et al, hereby incorporated herein by reference, discloses a debubbling device suitable for use in debubbling photographic compositions.
U.S. Pat. No. 3,432,691 to Shoh discloses an ultrasonic generator capable of operation at the parallel resonant frequency of an ultrasonic transducer.
Many delivery systems for delivering photographic compositions from a holding vessel to a coating station are equipped with inline devices for determining the presence of bubbles in the composition. Such a device is commonly known as a xe2x80x9cbubble detectorxe2x80x9d (BD), and the BD alarm signal is known as xe2x80x9cbubble detector activityxe2x80x9d (BDA). The intensity or frequency of the alarm signal is generally proportional to the size and/or number of bubbles passing through the BD and thus is an accepted measure of the relative effectiveness of a bubble elimination device (BED) upstream of the BD.
Because the removal of bubbles from flowing liquids can be critical to the quality of the products made with such liquids and the speed at which such products can be made, increasing the effectiveness of a bubble elimination device is always desirable.
It is therefore an object of the present invention to provide a method for increasing the effectiveness of a bubble elimination device in removing entrained bubbles from a flowing stream of liquid composition.
A further object of the present invention is to operate an ultrasonic bubble elimination device in accordance with an improved method to maximize debubbling capacity across a range of changing compositions.
Yet another object of the present invention is to provide a method and apparatus that is useful in providing bubble-free flow of liquid compositions, and especially for enabling the formation of uniform coatings of liquid compositions on moving webs.
Still a further object of the present invention is to provide a method for operating a debubbling apparatus wherein the debubbling apparatus will not suffer damage if operated without liquid present.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by operating an ultrasonic bubble eliminator at a constant output voltage at or near the parallel resonant frequency of the eliminator""s transducer as opposed to prior art bubble eliminators that are generally operated at constant power output. One benefit of operating at constant voltage, rather than constant power, is that the power dissipated in the transducer remains essentially constant, while the power transferred to the load may vary over a wide range. Thus, the apparatus is able to respond automatically to large changes in load. A second benefit is that the power to the transducer is automatically reduced when the load is removed from the ultrasonic horn, e.g., when the vessel is empty and the horn is coupled to air. High power applied to a horn without load can lead to damage or destruction of the transducer and/or horn.
In the apparatus used to practice the method of the present invention, an ultrasonic debubbler is connected to a variable-frequency RF generator capable of operating at or near the parallel resonance frequency of the debubbler transducer and at constant voltage. In the practice of the method, the debubbler is filled with degassed water and the generator is tuned to or near the parallel frequency of the transducer, typically about 40 kHz. Then the generator and debubbler are jointly calibrated to operate at a constant output voltage (preferably between about 200 volts and 400 volts) at a predetermined power (preferably from about 20 watts to about 40 watts) at a minimum ultrasonic load with the horn immersed in the degassed water. After establishing these settings, the debubbler is placed in service by being coupled inline into a delivery system for liquid composition, and the generator is operated at the established voltage setting. The power delivered to the transducer and horn is then spontaneously varied by the generator in proportion to the ultrasonic load on the debubbler. Experiments have shown such operation at constant voltage to be superior to prior art operation at constant power in removing bubbles from a flowing composition. The present invention is especially useful in the preparation of photographic emulsions for coating.